1. Field of the Invention
The present invention is broadly concerned with a low cost, improved pad-mounted double-fused vacuum switchgear of the type described in U.S. Pat. No. 4,083,028. More particularly, it is concerned with such improved switchgear making use of a specially configured dielectric tank in comination with a low cost fuse well assembly which permits substantial reduction in material and fabrication costs while also giving equivalent and in some cases improved electrical characteristics.
2. Description of the Prior Art
U.S. Pat. No. 4,083,028 to Haubein et al. describes a pad-mounted double-fused switchgear apparatus designed to achieve fully coordinated, high rated electrical protection for underground electrical distribution systems. The switchgear apparatus described in the aforementioned patent represents a significant breakthrough in the art which has achieved considerable commercial success. The patented switchgear apparatus broadly includes a tank adapted to hold a fluid dielectric material such as oil together with plural vacuum switchgear devices and respective, series-coupled double-fuse protective assemblies. Each of the latter includes a current limiting fuse (CLF) and a series connected expulsion-type fuse link.
Field experience with such switchgear has demonstrated its ability to handle both high and low level faults without catastrophic explosions notwithstanding the fact that the switchgear is of relatively small, low profile design.
During routine maintenance or refusing of switchgear in accordance with U.S. Pat. No. 4,083,028, it is recommended that the secondary or tap side bushings be disconnected, whereupon the respective fuse assemblies can be readily removed from the tank without fear of short circuiting or electrical shock. Despite these recommendations however, certain individuals have attempted to service the switchgear without properly disconnecting the tap side bushings. Under such unauthorized conditions, it is possible that as the fuse assemblies are withdrawn a short circuit can occur between the intermediate contacts of the fuse well assemblies and the obliquely oriented upper metallic castings of the fuse wells.
The patented switchgear apparatus also makes use of a pair of upright, essentially parallel sidewalls, with the tap side sidewall having a plurality (usually six) metallic tubular castings affixed thereto and defining the upper ends of the fuse well assemblies. Such a construction is inherently expensive because of the need to separately fabricate the castings and attach them by appropriate means to the tank sidewall. Moreover, the rectangular construction of the tank, together with the oblique orientation of the fuse well and fuse assemblies therein, creates a relatively large zone or open space directly beneath the tap side bushings which must be filled with oil; this oil is essentially irrelevant to the operation and electrical characteristics of the switchgear, and hence represents added weight and materials serving no useful purpose.